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comparison libtommath/bn_s_mp_sub.c @ 1739:13d834efc376 fuzz

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merge from main
author Matt Johnston <matt@ucc.asn.au>
date Thu, 15 Oct 2020 19:55:15 +0800
parents 1051e4eea25a
children
comparison
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1562:768ebf737aa0 1739:13d834efc376
1 #include <tommath_private.h> 1 #include "tommath_private.h"
2 #ifdef BN_S_MP_SUB_C 2 #ifdef BN_S_MP_SUB_C
3 /* LibTomMath, multiple-precision integer library -- Tom St Denis 3 /* LibTomMath, multiple-precision integer library -- Tom St Denis */
4 * 4 /* SPDX-License-Identifier: Unlicense */
5 * LibTomMath is a library that provides multiple-precision
6 * integer arithmetic as well as number theoretic functionality.
7 *
8 * The library was designed directly after the MPI library by
9 * Michael Fromberger but has been written from scratch with
10 * additional optimizations in place.
11 *
12 * The library is free for all purposes without any express
13 * guarantee it works.
14 *
15 * Tom St Denis, [email protected], http://libtom.org
16 */
17 5
18 /* low level subtraction (assumes |a| > |b|), HAC pp.595 Algorithm 14.9 */ 6 /* low level subtraction (assumes |a| > |b|), HAC pp.595 Algorithm 14.9 */
19 int 7 mp_err s_mp_sub(const mp_int *a, const mp_int *b, mp_int *c)
20 s_mp_sub (mp_int * a, mp_int * b, mp_int * c)
21 { 8 {
22 int olduse, res, min, max; 9 int olduse, min, max;
10 mp_err err;
23 11
24 /* find sizes */ 12 /* find sizes */
25 min = b->used; 13 min = b->used;
26 max = a->used; 14 max = a->used;
27 15
28 /* init result */ 16 /* init result */
29 if (c->alloc < max) { 17 if (c->alloc < max) {
30 if ((res = mp_grow (c, max)) != MP_OKAY) { 18 if ((err = mp_grow(c, max)) != MP_OKAY) {
31 return res; 19 return err;
32 } 20 }
33 } 21 }
34 olduse = c->used; 22 olduse = c->used;
35 c->used = max; 23 c->used = max;
36 24
37 { 25 {
38 mp_digit u, *tmpa, *tmpb, *tmpc; 26 mp_digit u, *tmpa, *tmpb, *tmpc;
39 int i; 27 int i;
40 28
41 /* alias for digit pointers */ 29 /* alias for digit pointers */
42 tmpa = a->dp; 30 tmpa = a->dp;
43 tmpb = b->dp; 31 tmpb = b->dp;
44 tmpc = c->dp; 32 tmpc = c->dp;
45 33
46 /* set carry to zero */ 34 /* set carry to zero */
47 u = 0; 35 u = 0;
48 for (i = 0; i < min; i++) { 36 for (i = 0; i < min; i++) {
49 /* T[i] = A[i] - B[i] - U */ 37 /* T[i] = A[i] - B[i] - U */
50 *tmpc = (*tmpa++ - *tmpb++) - u; 38 *tmpc = (*tmpa++ - *tmpb++) - u;
51 39
52 /* U = carry bit of T[i] 40 /* U = carry bit of T[i]
53 * Note this saves performing an AND operation since 41 * Note this saves performing an AND operation since
54 * if a carry does occur it will propagate all the way to the 42 * if a carry does occur it will propagate all the way to the
55 * MSB. As a result a single shift is enough to get the carry 43 * MSB. As a result a single shift is enough to get the carry
56 */ 44 */
57 u = *tmpc >> ((mp_digit)((CHAR_BIT * sizeof(mp_digit)) - 1)); 45 u = *tmpc >> (MP_SIZEOF_BITS(mp_digit) - 1u);
58 46
59 /* Clear carry from T[i] */ 47 /* Clear carry from T[i] */
60 *tmpc++ &= MP_MASK; 48 *tmpc++ &= MP_MASK;
61 } 49 }
62 50
63 /* now copy higher words if any, e.g. if A has more digits than B */ 51 /* now copy higher words if any, e.g. if A has more digits than B */
64 for (; i < max; i++) { 52 for (; i < max; i++) {
65 /* T[i] = A[i] - U */ 53 /* T[i] = A[i] - U */
66 *tmpc = *tmpa++ - u; 54 *tmpc = *tmpa++ - u;
67 55
68 /* U = carry bit of T[i] */ 56 /* U = carry bit of T[i] */
69 u = *tmpc >> ((mp_digit)((CHAR_BIT * sizeof(mp_digit)) - 1)); 57 u = *tmpc >> (MP_SIZEOF_BITS(mp_digit) - 1u);
70 58
71 /* Clear carry from T[i] */ 59 /* Clear carry from T[i] */
72 *tmpc++ &= MP_MASK; 60 *tmpc++ &= MP_MASK;
73 } 61 }
74 62
75 /* clear digits above used (since we may not have grown result above) */ 63 /* clear digits above used (since we may not have grown result above) */
76 for (i = c->used; i < olduse; i++) { 64 MP_ZERO_DIGITS(tmpc, olduse - c->used);
77 *tmpc++ = 0; 65 }
78 }
79 }
80 66
81 mp_clamp (c); 67 mp_clamp(c);
82 return MP_OKAY; 68 return MP_OKAY;
83 } 69 }
84 70
85 #endif 71 #endif
86
87 /* ref: $Format:%D$ */
88 /* git commit: $Format:%H$ */
89 /* commit time: $Format:%ai$ */